A quality of voice transceived via a communication network is affected by various factors such as environmental noise and an utterance level during utterance, performance of voice input and voice process at a voice transmitting side, a transmission quality, performance of voice output and voice process at a voice receiving side, environmental noise and a voice receiving level during reception, etc. Conventionally, a communication system of measuring the above-mentioned quality of voice is known, which is provided with a mobile telephone that is functioned as a communication terminal apparatus dedicated for measurements and in which a sound source file and an application for measurements of voice quality are implemented in advance, and a server for measurements of voice quality that is connected to a fixed-telephone communication network and in which a sound source file for measurements of voice quality are implemented in advance. In this communication system, a specialized staff responsible for measurements of voice quality operates the foregoing mobile telephone dedicated for measurements so as to perform an outgoing call to the server and notifies of a request to measure a voice quality. When receiving the request from the mobile telephone, the sever generates a voice signal based on the sound source file and transmits the generated voice signal to a communication terminal apparatus via the fixed-telephone communication network and a mobile telephone network. In the communication terminal apparatus, the voice signal received from the server is recorded and stored as a recorded audio file and a voice quality is measured by compared a voice signal of the recorded audio file with a voice signal of the sound source file. A measurement result of the voice quality is outputted, for example, as a MOS (Mean Opinion Scores) value standardized in Non-Patent Literature 1, Non-Patent Literature 2 and the like. The MOS value is a computed value corresponding to an average value of scores on five-grade evaluation (refer to Table 1) by a large number of persons based on comparison results between a voice to be evaluated and a voice of a reference sound source. In Non-Patent Literature 1 and Non-Patent Literature 2, an international standard method of mechanically realizing the foregoing computation of MOS values is described.
TABLE 1ScoreQuality5Excellent4Good3Fair2Poor1Bad